Polytrimethylene ether glycol (PO3G) and its uses have been described in the art. Preferred methods for preparation of a polytrimethylene ether glycol involve acid catalyzed polycondensation of 1,3-propanediol. For example, U.S. Pat. No. 6,720,459 and U.S. Pat. No. 6,977,291 disclose processes for preparation of a polytrimethylene ether glycol from 1,3-propanediol using a polycondensation catalyst, preferably an acid catalyst such as sulfuric acid.
Specifically, U.S. Pat. No. 6,977,291 describes a purification procedure for a crude polytrimethylene ether glycol obtained from an acid catalyzed polymerization process comprising: (1) a hydrolysis step to hydrolyze the acid esters formed during the acid catalyzed polymerization; (2) phase separation and water extraction steps to remove soluble acid catalyst, thereby generating an organic phase and a waste aqueous phase; (3) a base treatment of the organic phase to neutralize and precipitate the residual acid present; and (4) drying and filtration of the polymer to remove residual water and solids. It is clear from the disclosure that, when sulfuric acid is used as a catalyst to make polyether glycols from their corresponding diols, it is preferred to include a hydrolysis step because a substantial portion of the acid catalyst is converted to an ester (alkyl hydrogen sulfate).
U.S. 2005/0272911A1 and U.S. application Ser. No. 11/654,865 (filed Jan. 18, 2007) disclose polycondensation processes for preparing polyether polyols, including preparing polytrimethylene ether glycol from 1,3-propanediol, utilizing a polycondensation catalyst system that contains both an acid and a base. It is disclosed that the use of this catalyst system produces polyether polyol with a high degree of polymerization and low color under mild conditions. The purification processes utilize a hydrolysis step wherein water and organic solvent that has affinity for both water and the polyether polyol are used and subsequent separation into aqueous and organic phases. In one example of U.S. 2005/0227911A1, there is disclosed the use of calcium hydroxide to treat the organic phase after the phase separation.
U.S. Pat. No. 7,161,045 relates to a process of manufacture of polytrimethylene ether glycol comprising: (a) polycondensing reactant comprising a diol selected from the group consisting of 1,3-propanediol, 1,3-propanediol dimer and 1,3-propanediol trimer or mixtures thereof in the presence of an acid polycondensation catalyst to form polytrimethylene ether glycol; (b) adding water to the polytrimethylene ether glycol and hydrolyzing the acid esters formed during the polycondensation to form a hydrolyzed mixture containing the polytrimethylene ether glycol and the hydrolyzed acid esters; (c) adding an organic solvent that is immiscible with water to the hydrolyzed mixture to form an aqueous-organic mixture comprising (i) an organic phase containing polytrimethylene ether glycol and residual acid polycondensation catalyst from the polycondensing, and (ii) a water phase; (d) separating the water phase and the organic phase; (e) adding a base to the separated organic phase to neutralize the residual acid polycondensation catalyst by forming salts of the residual acid polycondensation catalyst; (f) separating the organic phase into (i) a liquid phase comprising the polytrimethylene ether glycol, the organic solvent and any residual water, and (ii) a solid phase comprising the salts of the residual acid polycondensation catalyst and the base which is unreacted; and (g) removing the organic solvent and the residual water from the organic phase to obtain polytrimethylene ether glycol. The organic solvent added in step (c) helps to promote phase formation and separation.
U.S. Pat. No. 7,157,607 discloses a process of manufacture of polytrimethylene ether glycol that is similar to that disclosed in previously incorporated U.S. Pat. No. 7,161,045, except that an organic solvent that is miscible with polytrimethylene ether is added to the hydrolyzed aqueous mixture.
U.S. Pat. No. 7,164,046 and U.S. application Ser. No. 11/599,861 (filed Nov. 15, 2006) disclose the addition of one or more water-soluble, inorganic compounds to the hydrolyzed mixture to promote phase formation and separation, wherein the water-soluble, inorganic compounds are selected from the group consisting of an inorganic salt and an inorganic base.
In the PO3G polymer manufacturing processes described above, it is commonly found that the polytrimethylene ether glycol polymers have residual color that results into a lower-quality polymer, not adequate for many of the polymer applications. The quality of 1,3-propanediol, the polymerization process conditions, and polymer degradation are most likely responsible for the polymer discoloration. In addition, the color of the final polymer can be affected by factors such as temperature of polymerization, oxidizing agents present in the reaction mixture, acidity, etc.
The presence of color is a major impediment to providing a high-quality polytrimethylene glycol polymer. Because a majority of the current PO3G processes involve high-temperature processing, discoloration could happen essentially at any step during the process, especially with a strong oxidizing agent (such as H2SO4) present in the mixture.
The present invention reduces color of the resulting polymeric PO3G, without substantially compromising polymer properties, preferably in a shorter cycle time and at a lower cost, by using a hydride compound at one or more stages of the PO3G.